This invention relates to fiber optic sensors and, more particularly, to marine seismic streamers using optical fibers for sensing changes in acoustic fields.
Marine seismic sensing devices are known that utilize discrete optical hydrophones which are assembled into marine seismic streamers. The discrete hydrophones use a pulsed laser to provide light to hydrophones made of optical fiber wound around mandrels. Pressure changes about a hydrophone cause deformations, which in turn cause phase and frequency modulation of light traveling through the fibers within each discrete hydrophone. Those changes are recorded as interference patterns produced at each discrete sensor. The individual interference patterns are coupled to a return cable to return to the shipboard for processing. Discrete optical hydrophones require a significant amount of fabrication, because each hydrophone must be spliced to optical coupler and return fibers, and the whole assembly encased and inserted into a hydrophone streamer skeleton. Marine seismic streamers of such individual sensors are bulky and expensive to fabricate.
Alternate types of optical hydrophone streamer systems are also known that utilize a streamer with discrete optical hydrophone sensors that operate by phase and intensity modulation of laser light input. Each sensor includes a mandrel-wound section of fiber coupled to two tails of optical fiber, each tail ending in an internal mirror. In this approach, light is reflected back and forth between the tails to produce phase and intensity modulation of the optical signal in response to sensed local acoustic pressure change. These two-tail systems have not been considered practical or economical for use in marine seismic streamers.
Such conventional optical sensing systems are limited in their application by cross talk effects. For example, if the width of the pulse is less than the round-trip optical propagation delay in each sensor element, the output obtained through the optical coupler consists of a series of N+1 pulses that are separated in the time domain. Apart from cross-talk effects, these pulses contain no direct interferometric information. Application of this pulse train to a compensating interferometer of optical imbalance 2L coherently mixes pulses obtained from consecutive reflectors, thus generating the interferometric outputs from each sensor element. Cross talk occurs between optical sensors due to multiple reflection paths. The cross talk manifests itself as side-bands in a heterodyne modulation and demodulation. In seismic acquisition, cross-talk of acoustic signals between sensors is highly deleterious to processing data. It is generally accepted that these crossfeed products must be kept below xe2x88x9290 dB in order to provide quality seismic data. To achieve this level of crossfeed the reflectivity of the mirrors would have to be so low that there would be inadequate returned optical power to process.
The present invention is directed to providing seismic optical sensor systems that overcome the limitations of existing systems.
According to one aspect of the present invention, an optical sensor system for seismic exploration is provided that includes a single mode optical fiber, an optical coupler, a pulsed laser, and a compensating interferometer. The optical fiber is mounted within a linear casing and includes an input end and an opposite terminal end. The optical fiber further includes a plurality of partially reflective internal mirrors incorporated into the optical fiber at predetermined spaced intervals within the optical fiber with each pair adjacent mirrors defining a long gauge, linear, acoustic sensor. The optical coupler is coupled to the input end of the optical fiber and includes a first and a second output port. The pulsed laser is optically coupled to the first output port of the optical coupler and is adapted to provide an optical pulse width equal to or less than twice the one way time of travel of optical energy between the predetermined mirror intervals of the optical fiber. The compensating interferometer is optically coupled to the second output port of the optical coupler for receiving optical energy reflected from the internal mirror and includes a first path and a second path. The second path of the compensating interferometer includes a time delay equal to the two way time of travel of optical energy between the predetermined mirror intervals of the optical fiber.
According to another aspect of the present invention, a optical sensor system for seismic exploration is provided including a single mode optical fiber, a separate optical coupler, a pulsed laser, and a compensating interferometer. The single mode fiber is mounted within a linear, pressure sensitive casing, to form a continuous linear acoustic sensor. The single mode fiber includes an input end, an opposite terminal end, and at least two spaced apart, two-by-two, ratio optical couplers positioned between the fiber""s input end and the fiber""s terminal end. Each ratio optical coupler includes an input fiber having a non-reflective terminal end and an output fiber having a reflective terminal end. The separate optical coupler is coupled to the input end of the single mode fiber and includes a first and a second output port. The pulsed laser is optically coupled to the first output port of the separate optical coupler and is adapted to provide an optical pulse width equal to or less than twice the time of travel of optical energy between the intervals between reflective terminal ends of the ratio optical couplers. The compensating interferometer is optically coupled to the second output port of the separate optical coupler for receiving optical energy reflected from the reflective terminal ends of the ratio optical couplers. The compensating interferometer includes a first path and a second path. The second path includes a time delay equal to the two way time of travel of optical energy between the reflective terminal ends of the ratio optical couplers.
According to another aspect of the present invention, a method of processing data obtained from a hydrophone streamer is provided for a hydrophone streamer that includes: a series of linear, continuous, long gauge, optical fiber hydrophones, wherein each optical hydrophone is bounded by a pair of internal mirrors within the optical fiber, wherein each hydrophone includes a terminal internal mirror for reflecting a first portion of a pulsed optical signal back through the hydrophone to the signal source and for reflecting a second portion of the reflected optical signal back through the fiber to an interferometer coupled to first, second and third photo detectors and to a three-by-three optical coupler. The method includes subtracting signals from the first and second photo detectors from one another, adding signals from the second and third photo detectors, and transforming the rectangular coordinate data in the added and subtracted signals to polar coordinate data.
According to another aspect of the present invention, a pulsed laser is provided that includes a laser having an output port, and an optical switch operably coupled to the output port of the laser.
According to another aspect of the present invention, an optical switch is provided that includes a first single polarization fiber, a polarization scrambler, and a second apolarization fiber. The polarization scrambler has an input port and an output port. The input port of the polarization scrambler is operably coupled to the first single polarization fiber. The second single polarization fiber is operably coupled to the output port of the polarization scrambler.